1. Field of the Invention
This invention relates to a technique for the utilization of light radiation energy such as solar energy. More particularly, this invention relates to a method for converting an aqueous solution into a gaseous fuel by the use of solar energy and the thermal energy thereof.
2. Prior Art Statement
The oil crisis in 1973 gave a powerful impetus to the development of solar energy and other forms of energy to take the place of oil and the research on energy saving techniques such as the utilization of waste heat. Various research has since been continued for effective utilization of light radiation energy such as solar energy. Among various methods for the utilization of radiation energy is a method which comprises converting a radiation energy such as solar energy into a chemical energy through the agency of a chemical reaction and storing the chemical energy for future use. This method enjoys various great advantages such as (1) the fact that the energy stored in the form of chemical energy does not decrease spontaneously during the storage, (2) the fact that the energy can be used whenever it is needed, and (3) the fact that the stored energy can be used in the form of heat at a higher temperature than that of the original heat source. As one of the reactions available for this method, the liquid-phase dehydrogenation reaction of 2-propanol has been known to the art. This reaction comprises mixing 2-propanol with a catalyst such as a metal complex and heating and exposing the resultant mixture to light and, consequently, produces acetone with evolution of hydrogen gas. The catalysts heretofore proposed for use in this reaction include Raney nickel [Shiro Kudo: "Lecture on Catalyst Engineering, Vol. 7," (1964) Chijin Shokan, p. 209], a RhCl.sub.3 -SnCl.sub.2 -LiCl type complex [H. B. Charman: J. Chem. Soc., Ser. B. 584 (1970)], Ru(CO)(OCOCF.sub.3).sub.2 (PPh.sub.3).sub.2 [A. Dobson et al: "Inorg. Chem.," 16, 137 (1977)], Rh.sub.2 Cl.sub.2 (CO).sub.2 (Ph.sub.2 PCH.sub.2 PPh.sub.2).sub.2 [Yasukazu Saito: "Catalyst," 22, 253 (1980)], a RhCl.sub.3 -SnCl.sub.2 type complex [H. Moriyama et al: J. Chem. Soc., Perkin II, 369 (1982)], an IrCl.sub.3 SnCl.sub.2 type complex and RuCl.sub.2 -SnCl.sub.2 type complex [T. Yamakawa et al: 5th Int. Conf. Photochem. Conversion and Storage of Solar Energy, Osaka (1984), p. 306]. This reaction, however, is disadvantageous from the economic point of view and in terms of the utilization factor of the solar energy because it is required to use in the catalyst therefor such expensive substances as Raney nickel, rhodium, and iridium, able to utilize only the short-wavelength portion of the solar light, for example, and able to proceed only to the stage of acetone and, therefore, store the energy only in an insufficient amount and induce the gasification of the aqueous solution to an insufficient extent.